(1) Field of the Invention
This invention relates to extremely lightweight foamed metals, particularly aluminum and alloys thereof, comprising a multiplicity of cells, and to a method of producing the foamed metal in which the multiplicity of cells are uniform in size. This invention relates also to a sound absorbing material having excellent sound absorption characteristics over a wide acoustic frequency range from low to high frequency and an electromagnetic shielding material having excellent shielding characteristics, which are based on the foamed metal.
The foamed metal has a thin film aggregate structure of a plurality of uniformly dispersed cells having an average diameter of from 2 to 10 mm, has an extremely light weight as represented by an apparent specific gravity of from 0.2 to 0.8, and has excellent sound absorbing and electromagnetic shielding properties. Accordingly, the foamed metal is extremely useful as constructional material, structural material and functional material haivng many funcitons.
(2) Description of the Prior Art
Hitherto, a variety of studies and inventions have been made concerning means for producing foamed metals, but there has not been practiced a method for economically producing an extremely lightweight foamed metal consisting of a uniform aggregate of cells, because of much difficulties involved. In order to produce a foamed metal, it has been attempted to use a foaming agent such as hydrous inorganic compounds and metal hydrides which decomposes at a temperature slightly higher than the melting point of a metal to be foamed, e.g., aluminum or an alloy thereof, and as a means for raising the viscosity of a molten metal, it has been attempted to blow air or the like into the molten metal or use an oxygen-containing chemical as a thickener. However, a satisfactory foamed metal has not yet been obtained by such means.
Basically, it is important to appropriately select a combination of the kind and amount of the thickener for quickly producing the molten metal with a proper viscosity and the kind and amount of the foaming agent for appropriate formation of foam. In addition, in the process of solidification of the molten foamed metal by cooling it is necessary to reduce the loss of gas generated through decomposition of the foaming agent, and in order to prevent differences between the bubbles in the inside part of the foamed metal and those at peripheral part of the foamed metal as well as prevent formation of cavities at the time of solidifications it is necessary to appropriately control the pressure inside the mold.
In addition, the recent development of electronic apparatuses has brought about a serious problem of electromagnetic wave interference. To cope with the wave interference, it has been attempted to thermally spray zinc or apply a conductive coating material to plastic casting of the electronic apparatus or to mix a conductive filler into the plastics. However, satisfactory results have not been obtained by such means.
Besides, though there have been used wood, synthetic resins, plywood, etc. as constructional material for walls, desks, cupboards and the like, all of these materials have both merits and demerits in view of flame retardancy, weight, strength, cost, etc. Constructional plates are, in many cases, desired to be light in weight, excellent in flame retardancy and capable of being produced at low cost.